At present, various types of thermocouple measurement systems, associated generally with temperature regulation systems, are known and put to use in various applications.
Thus, the regulation of temperature of a sample placed in a magnet of a nuclear magnetic resonance spectrometer is effected by an electronic regulator using a thermocouple for measurement of temperature in the immediate proximity of the specimen.
At the base of the magnet of the spectrometer, the thermocouple wires, surrounded by a shielding sleeve, are connected to an extension cord, called a compensation cable, to transmit the signal to the input of the regulator, often several meters distant.
As a thermocouple is sensitive to temperature differences, the reference junction, called the cold junction, which is to say the junction points between the two conductors of the thermocouple wires and the two copper conductors of the measurement circuit, must be at a known temperature to effect temperature measurement.
One way of proceeding consists in immersing the cold junction in an ice bath. This has the advantage of giving an output voltage of zero for 0.degree. C., rendering the use of a thermocouple table possible.
A more practical method, known as "electronic compensation of cold or junction soldering" consists in adding a compensation voltage to the thermocouple signal so as to avoid the use of a reference junction maintained at a constant temperature (ice or oven).
The known temperature regulator systems are generally provided with a compensation device for the cold junction integrated in their structure, which permits attenuating the measurement errors induced by variations of ambient temperature.
Nevertheless, this internal cold junction compensation of the known regulator systems is relatively imprecise. The maximum precision that can be achieved is at most 5/100ths of a degree per degree of variation of ambient temperature.
Moreover, the junctions between the wires of the thermocouple and the wires of the compensation cable are also sensitive to variations of temperature. Thus, the compensation of the two thermocouple wires, which for example can be respectively copper and constantan, is never exactly identical to that of the two wires of the compensation cable, thereby giving rise to the generation of a disturbing electrical signal which will be added to the usable signal provided by the measuring thermocouple.
There are also known autonomous casings for compensation of the cold junction of a thermocouple.
However, the precision of compensation achieved by these known casings is not superior to that previously indicated, by virtue of the fact particularly of the direct influence of the ambient temperature variations at the connections of the thermocouple wires on said casing by means of connectors.